Is a wealth tax a good idea?
Yes and no.
Administration 101: "Incentivize what you want more of and de-incentivize what you want less of".
As a revenue generating method, a general wealth tax ispretty much unworkable.
Not everything can be taxed, so what you will mainly be doing is motivating people to channel wealth into those items. With administration and the closing of loopholes, this method will probably be less efficient than many alternatives.
However, as motivators, taxes work just fine.
One form of wealth tax could be to on accumulated cash in bank accounts, to motivate your population to invest (if mountains of paper money was becoming a problem for your currency).
You can institute taxes on water, electricity, and gasoline to motivate people to use less of those things. Obviously you then you have to be careful not to tax the work force out of work mobility, or out of their homes.
The results are predictable enough that your average national economist can figure them out, and thus very safe to use as long as you have administrations that care more about results than showing off.
The advantages are two-fold. You get people to do what you want, and you can use the extra income to supplement your normal revenue generation; lowering other taxes or making long-term investments as necessary.
The only obstacle is that in democracies we don't really have a motivator for politicians to do their jobs and follow these simple rules, besides reelection. But that is another discussion.